Recently, radio frequency identification (RFID) technology has gained tremendous popularity as a device for storing and transmitting information. RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. “Passive” tags, in contrast, have no internal power source. Instead, passive tags derive power from the reader, and the passive tag re-transmits or transponds information upon receiving the signal from the reader. Passive tags have a much shorter signal range (typically less than 20 feet).
Generally, both categories of tags have an electronic circuit that is typically in the form of an integrated circuit or silicon chip. The circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna which communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the communication function with the reader.
On the other hand, a chipless RFID tag has neither an integrated circuit nor discrete electronic components, such as the transistor. This feature allows chipless RFID tags to be printed directly onto a substrate at lower costs than traditional RFID tags.
As a practical matter, RFID technology uses radio frequencies that have much better penetration characteristics to material than do optical signals, and will work under more hostile environmental conditions than bar code labels. Therefore, the RFID tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tags may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like.
The RFID antenna may be printed directly on the substrate using a conductive metal ink. Alternatively, metal fibers may be incorporated directly into the substrate. For example, one chipless RFID technology from Inkode Corp uses embedded aluminum fibers that are embedded into paper. The aluminum fibers must be cut to the appropriate wavelength (¼ wavelength) and be incorporated into the paper fibers as a furnish additive during the papermaking process. Therefore, the Inkode method is costly and tedious.
Although particulate metal materials may be used for printing RFID inks, the superior characteristics of nanoparticulate metal materials in ink applications yields a better product. Commonly used nanomaterials are gold, silver, nickel and copper, among others. Nickel has been used for conductive inks for a very limited extent because of its relatively low conductivity (about 4 times less than that of copper or silver). Gold and silver can provide good conductivity, but are relatively expensive. Copper provides good conductivity at a low price (about one percent of that of silver). Unfortunately, copper is easily oxidized and the oxide is non-conductive.
It is known that copper can be deposited by the Galvanic action of a metal higher than copper in the galvanic series on a solution of a copper salt. The Galvanic process uses a spontaneous chemical reaction between two reagents to generate electricity. The process involves electron flow from the anodic reagent to the cathodic reagent, resulting in the reduction of the electron acceptor reagent and the oxidation of the electron donor reagent. In general, copper will precipitate from a solution of a copper salt by the Galvanic action of any metal having a lower reduction potential than copper (typically described as a less noble metal). For example, zinc metal (standard reduction potential of −0.76V) and aluminum metal (standard reduction potential of −1.68V) have lower reduction potentials than copper metal (standard reduction potential of +0.34V). Hence, both zinc and aluminum metals are capable of precipitating copper metal from a copper ion salt solution.
U.S. Pat. No. 3,084,063 to Barnes et al describes in a method of using the galvanic process to deposit copper on a substrate, but Barnes' method involves injecting a fluid suspension of a precipitating metal in powder form into a confined turbulent stream of a solution of a cupric salt flowing under pressure, and then spraying the mixture toward a surface to be coppered in such a manner that droplets of the mixture arrive at the surface before there has been sufficient interaction within the mixture to precipitate appreciable amounts of metallic copper. As Barnes describes, under normal circumstances, deposition of copper commences approximately 20 seconds after mixing of the precipitating metal with the cupric salt solution. Therefore, the mixture must be sprayed before this period has lapsed.
Additionally,
Barnes does not mention use of nanoparticle metals.
Thus, there exists a need for a cheaper method for producing chipless RFID tags.